Industry Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012 segment	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information
Number of business segments									2
Segment Reporting Information, Profit (Loss)
Operating revenues	$ 433,046	$ 295,324	$ 470,355	$ 418,444	$ 288,148	$ 378,568	$ 330,399	$ 486,364	$ 1,617,169	$ 1,483,479	$ 1,578,534
Operating Income (Loss)	115,166	19,458	220,598	104,170	31,641	150,412	106,335	159,881	459,392	448,269	493,377
Depreciation, depletion and amortization expense									419,598	283,997	247,865
Interest expense									65,556	44,822	39,222
Income tax expense (benefit)									143,819	145,701	166,990
Captial expenditures									1,363,080	1,189,436	811,348
Identifiable assets	6,175,890				5,237,416				6,175,890	5,237,416	4,363,560
Property, plant and equipment, net	5,541,636				4,620,776				5,541,636	4,620,776	3,719,227
Oil and Gas Operations
Segment Reporting Information, Profit (Loss)
Operating revenues									1,165,580	948,526	958,762
Operating Income (Loss)									367,243	363,131	406,729
Depreciation, depletion and amortization expense									377,328	244,081	203,823
Interest expense									49,972	30,907	25,753
Income tax expense (benefit)									114,375	120,079	138,775
Captial expenditures									1,291,211	1,115,452	717,782
Identifiable assets	4,975,170				4,046,242				4,975,170	4,046,242	3,160,601
Property, plant and equipment, net	4,697,683				3,806,787				4,697,683	3,806,787	2,936,284
Natural Gas Distribution
Segment Reporting Information, Profit (Loss)
Operating revenues									451,589	534,953	619,772
Operating Income (Loss)									93,216	86,216	88,383
Depreciation, depletion and amortization expense									42,270	39,916	44,042
Interest expense									16,284	14,740	13,894
Income tax expense (benefit)									30,244	26,670	29,875
Captial expenditures									71,869	73,984	93,566
Identifiable assets	1,177,134				1,163,959				1,177,134	1,163,959	1,166,899
Property, plant and equipment, net	842,685				813,471				842,685	813,471	782,665
Eliminations and other
Segment Reporting Information, Profit (Loss)
Operating Income (Loss)									(1,067)	(1,078)	(1,735)
Interest expense									(700)	(825)	(425)
Income tax expense (benefit)									(800)	(1,048)	(1,660)
Identifiable assets	23,586				27,215				23,586	27,215	36,060
Property, plant and equipment, net	$ 1,268				$ 518				$ 1,268	$ 518	$ 278